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CO2: Don’t count on the trees

By Douglas Fox

FORTY-FIVE minutes’ drive from Panama City, followed by a 20-minute boat ride and a 30-minute trek into the rainforest, will bring you to some of the best-studied trees in the world. On a half-square-kilometre plot of land on Barro Colorado Island, the lives and deaths of precisely 208,387 trees have been tracked for 20 years, like patients in a clinical trial. In that time the trees have been monitored for signs of how tropical forests are responding to climate change. Now the news is in, and it isn’t good.

It has long been assumed that forests will be the “get out of jail free card” we need to help soak up spiralling carbon dioxide emissions. Much of that hope has been pinned on tropical forests, the so-called “lungs of the planet”, with some studies suggesting that elevated CO2 levels are allowing rainforests to grow more quickly, locking away the extra carbon in wood or soil mulch. Now measurements from the Smithsonian Tropical Research Institute (STRI) on Barro Colorado Island, and from other sites around the world are starting to suggest quite the opposite. Trees in these areas are starting to grow more slowly – a sign that they may already be suffering from climate change and might not be able to lock away our CO2 after all. “It is potentially very worrying,” says climatologist Rachel Warren from the University of East Anglia in the UK.

The idea that trees will help soak up our carbon emissions has been building for decades. In the early 1960s, when it was first noted that atmospheric CO2 …